As shown in FIG. 10 and as is widely known, when a group III nitride compound semiconductor such as gallium nitride (GaN) formed by crystal growth on a base substrate which is made of, e.g., silicon (Si), is cooled to ambient temperature, a number of dislocations and cracks are generated in the grown nitride compound semiconductor layer.
FIG. 11 is a schematic cross-sectional view showing a conventional semiconductor crystal formed on a Si substrate (a base substrate) by crystal growth. MOCVD is applied in a process of crystal growth of the semiconductor crystal. As shown in FIG. 11, a semiconductor crystal (e.g. GaN crystal) grown at a high temperature on a Si substrate (base substrate) by using a conventional technique has a reaction part, dislocations and cracks.
A conventional technique in which a semiconductor crystal consisting of a group III nitride compound semiconductor is grown on a base substrate and a free-standing semiconductor crystal is obtained independently from the base substrate is generally known as, for example, wet etching disclosed in a Japanese Patent Application Laid-Open (kokai) No. 7-202265 entitled “Manufacture of Group III Nitride Semiconductor” and a method comprising steps of growing a thick GaN (semiconductor crystal to be obtained) on a sapphire substrate by using HVPE or other process and removing the sapphire substrate by irradiating laser or polishing.